On Fri, Aug 05, 2022, Sean Christopherson wrote:
> Account and track NX huge pages for nonpaging MMUs so that a future
> enhancement to precisely check if a shadow page can't be replaced by a NX
> huge page doesn't get false positives. Without correct tracking, KVM can
> get stuck in a loop if an instruction is fetching and writing data on the
> same huge page, e.g. KVM installs a small executable page on the fetch
> fault, replaces it with an NX huge page on the write fault, and faults
> again on the fetch.
>
> Alternatively, and perhaps ideally, KVM would simply not enforce the
> workaround for nonpaging MMUs. The guest has no page tables to abuse
> and KVM is guaranteed to switch to a different MMU on CR0.PG being
> toggled so there's no security or performance concerns. However, getting
> make_spte() to play nice now and in the future is unnecessarily complex.
>
> In the current code base, make_spte() can enforce the mitigation if TDP
> is enabled or the MMU is indirect, but make_spte() may not always have a
> vCPU/MMU to work with, e.g. if KVM were to support in-line huge page
> promotion when disabling dirty logging.
>
> Without a vCPU/MMU, KVM could either pass in the correct information
> and/or derive it from the shadow page, but the former is ugly and the
> latter subtly non-trivial due to the possibility of direct shadow pages
> in indirect MMUs. Given that using shadow paging with an unpaged guest
> is far from top priority _and_ has been subjected to the workaround since
> its inception, keep it simple and just fix the accounting glitch.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>
Reviewed-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> ---
> arch/x86/kvm/mmu/mmu.c | 2 +-
> arch/x86/kvm/mmu/spte.c | 12 ++++++++++++
> 2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 53d0dafa68ff..345b6b22ab68 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3123,7 +3123,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> continue;
>
> link_shadow_page(vcpu, it.sptep, sp);
> - if (fault->is_tdp && fault->huge_page_disallowed)
> + if (fault->huge_page_disallowed)
> account_nx_huge_page(vcpu->kvm, sp,
> fault->req_level >= it.level);
> }
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 7314d27d57a4..52186b795bce 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -147,6 +147,18 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> if (!prefetch)
> spte |= spte_shadow_accessed_mask(spte);
>
> + /*
> + * For simplicity, enforce the NX huge page mitigation even if not
> + * strictly necessary. KVM could ignore the mitigation if paging is
> + * disabled in the guest, as the guest doesn't have an page tables to
> + * abuse. But to safely ignore the mitigation, KVM would have to
> + * ensure a new MMU is loaded (or all shadow pages zapped) when CR0.PG
> + * is toggled on, and that's a net negative for performance when TDP is
> + * enabled. When TDP is disabled, KVM will always switch to a new MMU
> + * when CR0.PG is toggled, but leveraging that to ignore the mitigation
> + * would tie make_spte() further to vCPU/MMU state, and add complexity
> + * just to optimize a mode that is anything but performance critical.
> + */
> if (level > PG_LEVEL_4K && (pte_access & ACC_EXEC_MASK) &&
> is_nx_huge_page_enabled(vcpu->kvm)) {
> pte_access &= ~ACC_EXEC_MASK;
> --
> 2.37.1.559.g78731f0fdb-goog
>
